In recent years, plasma processing apparatuses are used for treatments, such as deposition, etching, or asking, in a manufacturing process of semiconductor products. Specially, a plasma processing apparatus which generates a high-density plasma using a microwave is likely to be used because the plasma is stably generated in a high vacuum state at a relatively low pressure of about 0.1 mTorr (13.3 mPa) to a few Torr (a few hundreds Pa). Such plasma processing apparatus is disclosed in, for example, Japanese Published Unexamined Application Nos. H3-191073, H5-343334, H9-181052, 2003-332326, 2004-039972.
A common plasma processing apparatus using a microwave will be explained in summary with reference to FIG. 18.
A plasma processing apparatus 2 shown in FIG. 18 is provided with a placing table 6 for placing a semiconductor wafer W in a vacuum processing container 4. On the upper portion facing to this placing table 6, a disk-shape top plate 8, which transmits a microwave, is provided hermetically. On the side wall of the processing container 4, a gas nozzle 9 is provided for introducing a gas into the container.
On the upper face of the top plate 8, a planar antenna member 10 and a slow-wave structure 12 are provided. On the antenna member 10, a plurality of microwave ejection holes 14 are formed. A center conductor 18 of a coaxial waveguide 16 is connected to the center of the antenna member 10 to guide the microwave generated from a microwave generator 20 through a mode exchanger 22.
Then, the microwave is radiated from an ejection hole of the planar antenna member 10 while radially propagating in the radius direction of the antenna member 10. The radiated microwave is transmitted through the top plate 8 and introduced into the processing container 4. By this microwave, the plasma processing, such as etching or film forming on the semiconductor wafer W is applied using the plasma generated in a processing space S of the processing container 4.
In this case, a necessary gas is supplied from the gas nozzle 9 provided on the side wall of the processing container 4. For this reason, the disassociation degree of the gas is different in the outside of the wafer W which is located close to the nozzle 9, and the center portion of the wafer W depending on the time that the distributed processing gas is exposed to the plasma. As a result of this, there have been cases of an uneven etching rate or film thickness by the plasma processing a wafer face.
Therefore, a measure, such as that disclosed in, for example, the Japanese Published Unexamined Patent Application No. H5-345982, has been taken. That is, a gas channel is provided in the center conductor 18, which passes through the center of the coaxial waveguide 16, by forming a stick shaped center conductor that is hollow in the center. Also, the gas passage is provided so as to penetrate the top plate 8 to communicate with the gas channel of the center conductor 18. In this way, the process gas is directly introduced to the center of the processing space S.
However, in this case, the electric field strength inside the gas channel formed in the center of the top plate 8 is high, and the plasma in the processing space easily intrudes into the gas passage from the front end opening of the gas passage, thus there are cases where an abnormal electric discharge occurs in the gas passage. For this reason, there has been a problem of damaging the top plate 8 by excessive heat to the center of the top plate 8 due to this plasma abnormal electric discharge.
Therefore, forming the gas passage which extends in a radius direction to the center portion, onto the top plate 8 itself can be considered. However, this configuration can not be adopted because the electric field strength inside the gas channel is high, and the plasma in the processing space easily intrudes into the gas passage from the front end opening of the gas passage in this case as well, thereby the plasma abnormal electric discharge occurs.
Also, a configuration in which a shower head is prepared by assembling glass tubes having a plurality of gas ejection holes in a lattice shape and providing the shower head to the upper portion of the processing container, has been proposed. However, it is not preferable because the plasma abnormal electric discharge occurs in this case as well.